1. Field of the Invention
The present invention relates to an optical system and an optical apparatus, such as a digital still camera, a digital video camera, a film-based camera, a surveillance camera, and a projector.
2. Description of the Related Art
As an image pickup optical system used for an optical apparatus such as a digital camera and a video camera, there is known an image pickup optical system having such a high numerical aperture that an F-number is about 1.2 to 2.0 and an intermediate focal length that is a comparatively long focal length. This image pickup optical system is used widely for the portrait photography and indoor sport photography. Such an image pickup optical system is demanded for a small lens overall length (which is a distance from a first lens surface to an image plane), a small configuration, and high optical performance.
In general, a chromatic aberration, such as a longitudinal chromatic aberration and a lateral chromatic aberration, increases as the lens overall length becomes short and the optical performance deteriorates in the image pickup optical system. In particular, the chromatic aberration increases as the focal length becomes long in an intermediate telephoto lens and a telephoto lens which have a long focal length. Moreover, the chromatic aberration increases as the lens overall length becomes short.
On the other hand, a bright image pickup optical system having a high numerical aperture has a shallow depth of field, and a variety of aberrations strongly influence on the image quality. A more highly accurate aberration correction is required for the bright image pickup optical system. Accordingly, Japanese Patent Laid-Open No. (“JP”) 05-188286 discloses an intermediate telephoto image pickup optics system which has a high numerical aperture and a comparatively long focal length. JP 2005-321574 discloses a telephoto lens that includes a front unit on the object side including a lens made of an abnormal partial dispersion material such as fluorite so as to correct the chromatic aberration. The image pickup optical system disclosed in JP 05-188286 includes many lenses each made of a high refractive index and an intermediate dispersion glass material so as to correct a variety of aberrations such as a spherical aberration and a curvature of field. The telephoto lens disclosed in JP 2005-321574 includes many lenses each made of a low-refractive-index and low-dispersion glass material so as to correct the chromatic aberration.
The longitudinal chromatic aberration generally deteriorates when a high-refractive-index and intermediate-dispersion glass material having an Abbe number of about 40-60 and a refractive index of about 1.6-1.8 is used for a lens having a positive refractive power because that glass material has a negative abnormal partial dispersion. When a low-refractive-index and low-dispersion glass material having an Abbe number of about 70-90 and a refractive index of about 1.4-1.5 is used for a lens having a positive refractive power, a longitudinal chromatic aberration can be properly corrected because that glass material has a positive abnormal partial dispersion.
However, the lens surface (refractive surface) needs a high curvature for a desired refractive power because the refractive index is low. As a result, the spherical aberration and curvature of field are likely to occur. In order to obtain high optical performance over the screen, it is necessary to select a proper glass material, and to properly correct the chromatic aberration and other aberrations, such as a spherical aberration and a curvature of field.
In particular, in the intermediate focal length lens having a comparatively long focal length and a high numerical aperture, it becomes difficult to obtain high optical performance over the screen unless the chromatic aberration and other aberrations, such as a spherical aberration and a curvature of field, are well corrected.